BaseTool/VfrCompile: Fix potential memory leak issue

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=771

Cc: Eric Dong <eric.dong@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Dandan Bi <dandan.bi@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>

BaseTools/Source/C/VfrCompile/VfrUtilityLib.cpp

@@ -3504,6 +3504,10 @@ CVfrStringDB::SetStringFileName(IN CHAR8 *StringFileName)
     return;
   }
 
+  if (mStringFileName != NULL) {
+    delete[] mStringFileName;
+  }
+
   FileLen = strlen (StringFileName) + 1;
   mStringFileName = new CHAR8[FileLen];
   if (mStringFileName == NULL) {